UHT pasteurizer with regeneration

ABSTRACT

A pasteurization arrangement employs regeneration to bring raw milk or other liquid food product up to a few degrees below a UHT pasteurizing temperature. The product is first heated up by regeneration to about 175° F., and held at this temperature for about sixty seconds to denaturize proteins so that they do not deposit on the heat exchange walls of following stages. Then the product proceeds through a product-product regenerative heat exchanger where it emerges at about 264° and proceeds to a water-product heater to heat the product to 280° F. The water flows in counter current to the product. The temperature differential at any reference point in the heater and the regenerative heat exchanger is below 20°, and the ratio of rate of water flow to product flow in the heater is below about 3:1.

BACKGROUND OF THE INVENTION

This invention relates to the pasteurization of dairy products such asmilk, cream, and ice cream mix and other liquid food products, e.g.,fruit juices and soups. The invention is more particularly concernedwith a continuous-process pasteurization technique for heating milk (orother food product) to an ultra high temperature suitable for long shelflife, and which recovers most of the heat employed by regeneration, andwhere the pasteurized product is cooled by incoming raw product, whichin turn is heated to a temperature near the pasteurization temperature.The invention is also concerned with a technique which avoids a cookedor scalded taste in the food product.

Unprocessed milk and other raw food commodities are highly perishable,and also may harbor pathogenic microorganisms. Milk is a highlynutritious food, and thus also serves as an excellent growth medium formicroorganisms, most of which are capable of deteriorating or spoilingthe milk or milk products. This is the case also for fruit juices,broths, and many other liquid food products. Some microorganisms arepathogenic, and can present a threat to public health, and so it isimportant that the product is rendered completely free of pathogenicmicroorganisms. Therefore, milk and other liquid food products areprocessed to make them safe for human consumption and to reduce spoilageto the lowest level possible.

Pasteurization is a process for heat treating the milk or other foodprocess to kill these microorganisms. For milk, pasteurization requiresthat every particle of milk or milk product be heated to a temperature,for an adequate length of time sufficient to render it free ofpathogens, most notably Rickettsia and Mycobacterium tuberculosis. TheU.S. Public Health Service has published standards for time-temperaturerelations for pasteurization of milk and other milk products, and thesecan be at higher temperatures or longer times for products that maycontain added sweeteners or additional milk fat. Most modern dairiesemploy a continuous process pasteurization technique, rather than abatch process, even for highly viscous products such as ice cream mix.Typically, for most pasteurized milk, other than ultra-pasteurized orUHT pasteurized, a continuous process high-temperature, short time(HTST) technique is used, in which the milk is heated to about 165degrees F and then passes through a holding tube where the milk is heldat that temperature for a short time, e.g., sixteen seconds. These HTSTpasteurizers can employ either a plate heat exchanger, or “press,” inwhich parallel plates define flow channels for the milk and for heatingand cooling media, or can employ a tubular heat exchanger in which twoor more tubes of different diameter are arranged coaxially to defineflow paths for the milk and other heat transfer media.

In a HTST set up, cold raw milk (about 40 ° F.) is supplied from abalance tank into a regenerator section of the pasteurizer. Here the rawmilk is heated up by heat given up by pasteurized milk flowing incounter current direction through an opposite side of the regenerator.The raw milk leaves the regenerator and passes through a positivedisplacement timing pump that delivers the milk under pressure tofurther stages. The pre-heated raw milk is then forced through a heatersection where a heating medium, e.g., water or a food grade syntheticproduct, heats the milk up to a temperature of at least about 162° F.The milk, having reached this pasteurization temperature, flows througha holding tube or timing tube, where the milk is held at thistemperature for a predetermined time, e.g., at least 16 seconds. Thevelocity of the milk product is determined by the speed of the timingpump, the diameter and length of the holding tube and other sources ofsurface friction. After passing temperature sensors at the end of theholding tube, the milk flows past a flow diversion device, which isintended to return the milk product through a divert line to the balancetank if the temperature of the product is below the presetpasteurization temperature. Properly heated milk will continue forward,and passes through the pasteurized side of the regenerator where it iscooled by the incoming raw milk. The milk can be cooled further to about40° F., and processed for bottling, packaging, cheesemaking, or otheruse. A homogenizer may be used as the timing pump just described, or mayconstitute additional auxiliary equipment, typically at the stage wherethe milk or milk product has been heated. Booster pumps are also presentin the flow path to ensure correct pressure and flow relationships. Forany continuous pasteurization technique, it is important to maintain ahigher pressure on the pasteurized side of any product-to-product heatexchanger, such as the regenerator, so that in the event there is anypinhole leak the flow of milk is away from the pasteurized side. Thisprevents contamination of the pasteurized milk with raw milk. Thepressure differential is maintained using the timing pump, and otherpumps and pressure controllers, and by ensuring that there is anyvertical rise in the product flow path is kept within limits.

The purpose of the regenerator is to save energy used in heating andcooling the food product during pasteurization, by using the heatcontent of the pasteurized product to pre-heat the incoming cold rawmilk. The efficiency, i.e., percent regeneration, is equal to the ratioof the temperature increase in the raw milk due to regeneration to thetotal temperature increase. For example, for cold milk drawn from thebalance tank at 40° F., heated through the regenerator to 145° F., andthen heated to a final temperature of 165° F., the efficiency would be84%, i.e., an 84% regeneration:$\frac{\left( {145 - 40} \right)}{\left( {165 - 40} \right)} = {\frac{105}{125} = {84\%}}$

As the cost of energy is a significant consideration in the overall costof processing the milk product, it is desirable to keep the amount ofregeneration as high as possible, and thus to reduce the cost of addingheat at the heater stage.

Ultra high temperature treatment of the food product, i.e., UHTpasteurization, involves heating the liquid food product continuously,and ensuring that every particle of the milk or other food product hasbeen held at the predetermined ultrahigh temperature for a minimumlength of time. The UHT technique can be incorporated into asterilization technique, in which the product is heated to a temperatureof 240° F. or above, and is held for a corresponding holding time toensure that the microorganisms and their spores in the product aredestroyed. Then the sterilized product is packaged aseptically, andaseptically sealed. The intention here is to permit the liquid foodproduct to be stored at room temperature indefinitely without spoilagedue to action of microorganisms. However, the process of ultra highpasteurization processing may alter the flavor or desirable color ortexture of the product, and may result in a “cooked” or scalded flavorin the product.

A vacuum treatment is sometimes employed to remove as much of theundesirable flavor components as possible from the product. In thevacuum process, milk is first heated to the desired temperature, andthen is passed into a chamber in which the pressure has been reduced bya partial vacuum. The pressure in the chamber is low enough to cause thevolatile flavor components to vaporize, and these are then evacuatedfrom the chamber. Some of the water in the product may be evaporated aswell. The vacuum treatment will also reduce any flavor components thatresult from the cows' ingestion of weeds or flavor-producing feedcomponents.

In addition to the public health and spoilage issues addressed bypasteurization, it has been discovered that some proteins and enzymes inmilk that will bring about chemical changes in the stored product can bealtered or removed, i.e., denaturized, by heat treatment.

A continuous flow milk sterilization process is described in U.S. Pat.No. 3,567,470, in which raw milk is passed though counterflowregenerators to a pasteurizer and then is passed though counterflow heatexchangers where its temperature is raised to a sterilizationtemperature. However, in this process, there is a rather hightemperature differential between the milk and the heating medium in theultra high temperature heat exchangers used for sterilization, so thatthe temperature differential between the milk and the heating medium atany given point of reference is always above 20 degrees and may reach 40degrees F. This means that the heating medium (steam) entering the heatexchanger has to be 300° F. to raise the milk to a sterilizationtemperature of 270 to 280° F., and the medium leaving is at 280° F.where it meets the product entering at 240 to 260° F. The processed milkin this system has to pass through a pair of vacuum tanks to remove airand entrapped volatile gases in an attempt to remove the objectionableburnt flavor that is associated with the sterilized milk.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for UHTpasteurization of liquid food products that avoids the drawbacks of theprior art.

It is another object to provide a process for UHT pasteurization thatmaintains the temperature differential in the heater stages at a levelthat maximizes the regeneration efficiency and also avoids off flavors,i.e., cooked or scalded flavors, in the liquid food product.

It is a further object to provide an arrangement that denaturizesproteins and similar components of the food product so that they do notdeposit on walls of the heater in the ultra high temperature stage or inthe regenerator.

It is another object to provide an arrangement that can convert anexisting pasteurizer, i.e., HTST pasteurizer (163 to 175° F.) which mayhave an overall regenerative efficiency of 90% to a UHT or asepticpasteurizer (280° F.) with an overall regeneration of over 85%.

A related object is to provide an arrangement that can convert a dairyproduct high temperature pasteurizer to a UHT pasteurizer withoutrequiring alterations of the existing pasteurizer.

It is yet a further object to provide a UHT pasteurizer in which theproduct is brought up to a sterilization temperature (e.g., above 250°F.) rapidly and efficiently.

Another object is to pasteurize milk and other dairy products attemperatures up to or in excess of 280° F. without loss of protein thatdeposits on heat transfer surfaces.

According to one aspect of the invention, a process for ultrahightemperature pasteurization of a liquid food product employs a firstproduct-to-product regenerative heat exchanger, a first heater stage, asecond product-to-product heat exchanger, and a UHT heater stage. Theliquid food product is supplied from a source, e.g., as balance tank,through a raw-product side of said first regenerative heat exchanger topre-heat same. Then the preheated liquid food product exiting the firstheat exchanger is heated, as need be, to a predetermined intermediatetemperature suitable for denaturizing proteins in the liquid foodproduct, and the product is held at that intermediate temperature, e.g.,by flowing through a timing tube for a predetermined time suitable fordenaturing the proteins. The liquid food product enters a raw-productside or pathway of the second regenerative heat exchanger to preheat thesame from the intermediate temperature to a temperature near the UHTpasteurizing temperature. The liquid food product exiting the secondheat exchanger flows into a medium-to-product heater in the UHT heaterstage where the liquid food product up to the UHT temperature. Here, theUHT heater stage employs a heating medium (water or a food grade glycol)that flows in counterflow to the product through the heater. The liquidfood product exiting the heater is held at the UHT temperature for apredetermined length of time, which may be a few seconds. Then theliquid food product returns through a pasturized side of the secondregenerative heat exchanger in counterflow to the liquid food productflowing in the raw side to warm the product flowing in the raw side, andcontinues to flow through a pasteurized side of the first regenerativeheat exchanger in counterflow to the liquid food product flowing in theraw side thereof to heat the product flowing in the raw side. Thepasteurized product may be further cooled, and sent to a fill tank whereit is ready for filling bottles or containers, or for other processing.In this invention, the temperature differential in the medium-to-productheater between the liquid food product and said heating medium at anypoint of reference in the heater is less than 20 degrees F. In afavorable embodiment, the temperature differential between the enteringproduct and leaving medium can be about 15 degrees, and that between theleaving product and the entering medium may be only a few degrees, e.g.,5 degrees. The temperature differential between the raw product leavingthe second stage regenerative heat exchanger and the pasteurized productentering is also below 20° F. The volume rate of flow of the mediumthrough the UHT heater should be at most about three times the rate offlow of the liquid food product, and the volume flow rate ratio may be2:1.

In the preferred embodiment, the food product is heated to anintermediate temperature of substantially 175° F., and the associatedtiming tube holds the milk at that temperature for sixty seconds. Thisensures that the milk proteins are suitably denaturized so that they donot deposit on the hot surfaces of the second regenerative heatexchanger or of the UHT heater. A homogenizer may be used as the timingpump at this intermediate heating stage.

In some embodiments, the regenerative UHT pasteurizer may be anarrangement designed and constructed entirely for that purpose. In otherembodiments, the UHT pasteurizer may be an attachment that converts anHTST pasteurizer to a UHT pasteurizer.

The arrangements of this invention may be used for whole milk, skimmilk, cream, or other dairy products such as flavored milk or ice creammix. The arrangements of this invention may also be used for processingorange juice, grapefruit juice, apple juice, or other fruit juices.Favorably, the equipment is of a tubular heat exchanger design, and maybe cleaned-in-place with a minimum of down time. A triple-tube heatexchanger that is suitable for use in various possible embodiments isdescribed in Feldmeier U.S. Pat. No. 3,386,497, which is incorporatedherein by reference.

The above and many other objects, features, and advantages of thearrangements and techniques of the present invention will becomeapparent from the ensuing detailed description of preferred embodimentsof the invention, when considered in connection with the accompanyingDrawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a generic diagram for explaining the UHT pasteurizationtechnique of this invention.

FIG. 2 is a schematic view of a UHT pasteurizer of one embodiment of theinvention.

FIG. 3 is a schematic view of a UHT pasteurizer of another embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To pasteurize milk legally, the milk must be heated to a minimum 163° F.(72.8° C.) and held at that temperature for at least 15 seconds. Milkproduct with a fat content of 10% or more must be heated to at least168° F. and held for at least 20 seconds. There are alternativetime-temperature relations, where the product can be heated to a highertemperature for a shorter period. One example of this is ultra hightemperature pasteurization, or UHT, defined as heating the milk productto 280° F. and holding at that temperature for a minimum of two (2)seconds. At the present time, the majority of the milk in the U.S. thatis pasteurized and bottled is heated between 163° F. and 175° F. andthen cooled below 40° F. The equipment used to attain suchpasteurization uses either a tubular heat exchanger or a plate type heatexchanger or press. These heat exchangers generally consist of threesections: a heater, a regenerator, and a cooler. The heating medium isusually water a few degrees higher than the pasteurization temperature.The cooling medium is typically ice water or food grade propylene glycolbeing at a lower temperature than the desired milk temperature. Theregenerator stage or section enables the raw milk to be heated by thepasteurized milk, and the pasteurized milk to be cooled down by the rawmilk. The regenerator is considered product-to-product, that is, themedium on both sides is the milk. The pasteurized milk is on one side ofthe plate or in one tubular pathway, while the raw milk is on the otherside. In the case of a tubular heat exchanger, an inner tube is mountedcoaxially within an outer tube to define counterflow paths for the rawand pasteurized milk. The raw milk is either on the inside or outside ofthe tubing with the pasteurized milk on the opposite side flowing incounter current.

For ultra-high temperature (UHT) pasteurization, there are two methodspresently in use. One method employs steam that is injected into themilk in a heater section, and then the milk goes through a vacuumchamber to remove the condensed water vapor and at the same time coolthe milk before it enters a plate or tubular regenerator and finalcooler. The percent regeneration with this type of arrangement is lessthan 75%. The other method in use employs tubular heat exchangers forthe heating section, and tubular or plate hat exchangers forregeneration and cooling. In these arrangements, the regenerators arenot product-to-product, but rather water-to-product. Circulating watercools the pasteurized product in one heat exchanger. This water whichhas gained heat is used in a second heat exchanger to preheat the rawproduct. The maximum practical total regeneration with this type ofregeneration is only about 80%, and typically below 75%. All thepractical UHT and aseptic dairy product pasteurizers in the U.S. todayutilize water-to-product regeneration. When the product emerges from theregenerator, it is pumped through a tubular heat exchanger at a veryhigh velocity (i.e., 18 feet/second) and consequently at a very highpressure drop through its tubes. The conventional thinking is that tominimize heat flavor to the product, the residence time of the productthat is being heated up from approximately 220 ° F. to 280° F. should beas short as possible. Also, the current thinking is to pump milk at veryhigh velocities to minimize the milk solids being deposited on theinside walls of the plates or tubes or other heat transfer surface.These deposits negatively affect the length of operation before havingto flush and clean. Homogenization is carried out after the UHT stage,that is, after the product has been heated up to 280° F. and then hasbeen cooled rapidly to approximately 175° F.

The challenge facing the dairy industry and the fruit juice industry isto address the need for a pasteurizing system that can be used for ultrahigh temperature pasteurization, which has a high regenerativeefficiency and produces pasteurized product without noticeable flavordegradation. The pasteurizer arrangement should have a capability oflong production runs, i.e., 8 to 10 hours, while maintaining an overallregeneration efficiency of 80% to 90%. The temperature of pasteurizationshould be adjustable so that the system can be used for aseptic or UHTpasteurization of milk, pasteurization of viscous dairy products such asice cream mix or heavy cream, while maintaining a regenerationefficiency of up to 75%, or for pasteurizing fruit juices and pulp.

The arrangement has to employ differential pressure controls so that anypinhole that might occur would permit any leakage of the milk to be inthe direction of the pasteurized side to the raw or unpasteurized sideso that there is no contamination of the pasteurized milk. Ideally, thearrangement should avoid vacuum chambers and additional pumps, and stillproduce milk with an acceptable flavor. The UHT or aseptic system mustbe sterilized prior to processing the milk or other liquid food productat over 250° F. for 20 to 30 minutes. All pipelines that conduct theproduct must be sterilized.

In a conversion of an existing pasteurizer to a UHT pasteurizer, the UHTsection can be installed at the point where the dairy product emergesfrom a legal flow diversion valve. The product flows from that point toa centrifugal pump. A vacuum breaker is installed between the flowdiversion valve and the pump so that the pump does not influence thevolume that exits the flow diversion valve. The product is then pumpedthrough a double tube regenerator or triple tube regenerator of the typedescribed in Feldmeier U.S. Pat. No. 3,336,497, and then proceeds to thesuction side of another centrifugal pump, where the product is pumpedthrough another double tube, triple tube, or multi-tube heat exchanger.The tubes that carry the product have heated water surrounding it orthem and the water is flowing in counter current to the product. The hotwater is just a few degrees above the UHT temperature, that is, 289° F.for a product temperature of 285° F. Within the water-to-product heatexchanger, the water volume flow rate is higher than the product flowrate, but less than three times the product flow rate. The heatedproduct flows through a holding tube where the UHT temperature ismaintained for a predetermined period of time, e.g., two seconds, andthen the product passes into the return or pasteurized side of theregenerator.

Once the product reaches 280° or more it is held for two seconds andthen flows counter currently through the above-mentioned regenerator.Within the regenerator, the ultra-pasteurized product pressure will behigher than that of the raw product that has not been heated to 280° F.Then the product exits this regenerator and is passed through thepasteurized product side of the existing pasteurizer regenerative heatexchanger, to be cooled. Note that the unpasteurized product bypassesthe regenerator during start up. By law, milk is not allowed to flow inthe unpasteurized side of the regenerator unless the pasteurized productis at a pressure higher than either of the raw product pumpcapabilities. After the milk emerges from the first regenerator, aholding tube or chamber maintains the milk at a predeterminedtemperature (e.g., 175° F.) for a period of time (30 seconds to 60seconds) to denaturize the milk solids so that they do not deposit onthe sides of the heat exchanger tubes.

With this UHT conversion, the ultra pasteurized milk has an acceptabletaste, either as white milk or as another dairy product. Anyobjectionable flavor that might be present immediately afterpasteurization dissipates and in not noticeable later, especially afterthe product has been in a package for about eight hours.

The system can run continuously over 8 hours, as the residence times inthe legal holding tube and the auxiliary holding tube, plus theresidence time in the regenerator, will denaturize the product so thatthe protein does not precipitate out and clog the tubes or restrict theflow. With this conversion, 40% cream and other viscous product will berun at about 50% capacity as the existing system would most likely beunable to handle a full volume of the viscous product, and would limitthe capacity of the UHT conversion section.

The system can also be constructed as a complete, self standing UHT oraseptic pasteurizer, using a double tube or triple tubeproduct-to-product regenerator. A double tube or multi-tube heatexchanger is used in the water-to-product heat exchanger and is designedso that the product velocity need not exceed about 8 to 9 feet persecond, and can typically be at 6 feet per second. Generally theresidence time from the time the product is homogenized until it reachesthe final temperature of 280° will be about two minutes. The velocity inthe heater section will be about six feet per second, with a pressuredrop of only about 20 PSI. This results in a large energy saving oversystems now employed that have velocities in the heating sections ofapproximately eighteen feet per second with very short residence times,i.e., less than 20 seconds. The overall regeneration efficiency will beabout 90%.

The UHT pasteurizer can operate continuously for over 8 hours withoutprotein build up to the point that the heat exchange rate is negativelyaffected. This system will also pasteurize viscous products at the sameflow rate as milk, providing the surface area in the heater section islarger than in a milk pasteurizer system. The percentage of regenerationin this system would be less for viscous products than for milk (orjuices).

After the first regenerator, the milk is held at the intermediatepasteurizing temperature for one or two minutes to denaturize theproteins. The holding tube for this would preferably be in the nature ofa double tube, triple tube, or multi-tube. In the case of an ice creammix or other viscous products, heat may be applied to bring thetemperature up to a suitable homogenization temperature. Prior toprocessing the product, heat will be applied to the circulating water tosterilize the raw side of the regenerator and to assist the heatingsection to bring the entire system up to standardization.

With reference to the Drawing, FIG. 1 is a schematic to explain thegeneral principles behind the pasteurization technique of thisinvention. Here, raw milk, i.e., milk that is to be pasteurized, issupplied to a balance tank 12 from a supply line (not shown), and themilk is drawn from this tank and pumped by a booster pump 14 through theraw milk side of a first stage regenerative heat exchanger 16. The milkon the raw side is heated up by the returning pasteurized milk on thepasteurized side of the heat exchanger, which in turn is cooled by theraw milk. The milk in the balance tank is kept at about 40° F. or below,and is heated up in the regenerative heat exchanger 16 so that at aleaving point 18 the milk is at an intermediate temperature of about160° F. A homogenizer 20, which may be used as a timing pump, pumps theheated milk at a predetermined rate through a supplemental heater thatbrings the milk to a higher intermediate temperature of 175° F. The milkpasses through a holding tube 24 where the milk is held for apredetermined minimum time (e.g., 60 seconds) at 175° to denaturizes themilk proteins so the milk solids to not precipitate out and coat thewalls of the following stages.

The milk then enters the raw side of a second stage regenerative heatexchanger 26, where the milk flows in counter current with pasteurizedmilk that is returning on the pasteurized side, so that the pasteurizedmilk is cooled and the raw milk is heated up to a temperature near theUHT pasteurizing temperature. In this example, the milk leaving the rawside of the second stage regenerative heat exchanger 26 has been heatedto a temperature of 264° F. and the pasteurized milk entering thepasteurized side is at the UHT temperature of 280° F.

The milk from the raw side of the heat exchanger 26 passes into a heater28 that brings the milk temperature up to the UHT temperature of 280°.In this case, the heater 28 is a tubular heat exchanger, which can be amulti-tube tubular unit, with hot water being the heating medium andflowing in countercurrent with respect to the milk (or other foodproduct). A water circulating portion of this embodiment employs a watercirculating pump 30 and a heater 32 that brings the water to atemperature a few degrees above the UHT temperature, and here to 284° F.The water entering the heater 28 is supplied at 284° F. and after givingheat off to the milk the water leaves the heater at 279° F. The flowrate of the hot water is higher than the flow rate of the milk throughthe heater 28, but is kept below about three times the flow rate, andfavorably about two times. This results in the temperature differentialbetween the food product and the heating medium being below twentydegrees F at any reference point within the heater, and in this example,the temperature differential is only four to six degrees between theentering water and the exiting milk, and only about fifteen degreesbetween the entering milk and the exiting water.

The milk leaving the heater 28 passes through a holding tube 34 where itis held at the UHT temperature (280°) before entering the return side,i.e., pasteurized side, of the second heat exchanger stage 26. It shouldbe noted also that at this stage, the temperature differential in theregenerative heat exchanger 26 between the pasteurized product enteringand the raw product leaving (280° minus 264° F.) is below 20 degrees,and the temperature differential between the pasteurized product and theraw product at all reference points within the second stage heatexchanger stage is below 20 degrees.

The pasteurized milk that leaves the second stage regenerator 26 thenpasses through the pasteurized side of the first stage regenerative heatexchanger 16, and leaves at about 60° F. The pasteurized product thenpasses through a cooler 36, i.e, a tubular heat exchanger where icewater or propylene glycol, for example, is used as the cooling medium,where the milk is cooled again down to 40° F. or below, and passesthrough a diversion valve 38 to a filler tank (not shown). A diversionline 40 returns product to the tank 12 when the valve 38 opens. Variousother diversion lines and valves, sterilization water heaters, and thelike, are not shown here, but would be understood to be present in anypractical design, and their construction and operation would be wellunderstood by persons in this field.

In some cases, i.e., where the product is whole milk or skim milk, thetemperature at the point 18 leaving the first regenerative heatexchanger may be sufficient for homogenization and denaturization, inwhich case the heater 22 could be omitted. The hold time in the holdtube 24 also depends on the nature of the product and the intermediatetemperature at that point. In some cases, a vacuum tank may be used inplace of the hold tube 26 for the purpose of removing onion or otherdisagreeable flavors from the milk, and also to hold the milk at theintermediate temperature for the time necessary.

The velocity of the milk through the system should be below about ninefeet per second and normally does not need to exceed about six feet persecond, especially in the second stage regenerative heat exchanger 26and in the medium-to-product heat exchanger 28.

One practical embodiment of this invention is shown in the arrangementof FIG. 2, which is a self-contained or dedicated UHT or asepticpasteurizer. Fresh milk is supplied to a balance tank 112 and the milkis pumped from the balance tank by a booster pump 114 through the rawmilk side of a regenerator 116, which may be a two-tube or three-tubeheat exchanger. This is a product-to-product heat exchanger in whichpasteurized milk is flowing in counter current through a pasteurizedflow path, so that the pasteurized milk warms up the raw milk, and theraw milk cools the pasteurized milk. At a mid portion of theregenerator, a take off 117 the milk has reached an intermediatetemperature, and then the milk passes from the take off 117 though abooster pump 118 to a holding tube 119 where the milk is maintained fora predetermined time, e.g., sixty seconds, to denaturizes the milksolids. If the milk at the take off 117 is at a sufficient temperature,e.g., 175° F., then no additional heat is needed at this point. However,in this example, the milk at this stage is 160° F., and in this case thehold tube 119 can be preceded by a heater stage to heat the milk orother product up to 175°. Following the hold tube 119, a homogenizer 120breaks up fat globules in the milk and also serves as timing pump. Thena booster pump 121 flows the milk back into a second portion of theregenerator 116, where it is again preheated, here to a temperature justbelow the predetermined UHT temperature. In this embodiment, the milkleaving the raw side of the regenerator 116 is at 264° F. The preheatedproduct flow past a diversion valve or valves 124 (diversion lines notshown) to another booster pump 125, from which the milk flow through aUHT heater 126. In the heater 126, the milk is heated from 264° to 280°.This is a medium-to-product heat exchanger, which may be a multi-tubetubular heat exchanger, with the heating medium being water flowingcounter current to the milk or other dairy product. A hot watercirculating unit uses steam to heat the water to a temperature a fewdegrees above the UHT temperature, here 284° F. The water entering theheater is at a temperature of 284° and the water leaving is 279°, so thetemperature differential throughout the heater is between four andfifteen degrees. The milk or other dairy product has a flow rate ofabout 6 feet per second, and the volume rate of flow of hot watercompared to the volume rate of flow of the product is below 3:1,typically closer to 2:1.

The product leaving the heater 126 then passes through a hold tube 128,where the temperature of 180° is maintained for a predetermined holdtime, here two seconds, and then the milk is considered ultrapasteurized, and is microbe free. The pasteurized product then returnsthrough the pasteurized flow path in the regenerator 116, where it iscooled by the raw product down to about 60° F. This is followed by acooler stage 130, where the milk is cooled by ice water or an approvedfood-grade medium such as propylene glycol, and leaves at about 40° F.The cooled pasteurized milk flows to a diversion valve 132, and thenceto a filler tank. A divert line 133 leads back to the balance tank 112.A divert line heater 134 is used for heating sterilizing water for thedivert line, and other water heaters 136 are used for heatingsterilizing water to about 250° F. at other portions of the pasteurizer.These can be steam-to-water heat exchangers. A regenerator bypass line135 may be used at start up.

Pressure differentials have to be maintained throughout the system sothat the pasteurized side at any given point is at a positive pressure,and any pinhole leaks would result in flow of pasteurized product to theraw side, and not vice versa. At all points in the regenerator 116 andheater 126, the flow rate is only about six feet per second, and thetemperature differential between the one side and the other at anyreference point is below twenty degrees.

With the set up as described there is a high rate of regeneration, inthis example, 87%. If the intermediate temperature at the take off 117does not need to be boosted by an additional heater to 175°, then theregeneration efficiency can be over 90%.

FIG. 3 illustrates an embodiment in which a standard HTST pasteurizer isconverted to a UHT pasteurizer by adding additional regeneration andheating stages. Here, an existing HTST pasteurizer 210 draws milk from abalance tank 212, and pumps it using a booster pump 214 through the rawside or raw flow path of a regenerator 216. The raw milk is heated bythe returning pasteurized product flowing countercurrently, and exits atan elevated temperature. The raw product that exits the regenerator 216flows through a heater 218, where it is heated to a predeterminedhigh-temperature pasteurization temperature, e.g., 175°, and then goesto a holding tube 220, where it is maintained for a minimum time (e.g.,sixty seconds) for denaturing the protein in the product. It may benoted that a de-aerator or vacuum chamber at this point will alsoaccomplish some denaturization of proteins, as there will be a holdingtime in such a chamber. The holding tube may be in the form of adouble-tube, triple-tube or multi-tube which may be employed for heatingice cream mix or other viscous products. After this denaturizingtreatment, the UHT section can handle the product at the same capacityas for milk. A double, triple, or multiple tube holding tube can also beused to heat water during sterilization. After the holding tube 220, acentrifugal pump 221 flows the product through a homogenizer 222, whichmay also serve as timing pump, and the product exiting the homogenizerpasses through a legal holding tube 224 where the product is held for apredetermined HT holding time (e.g., 15 seconds). Then the productpasses to a set of flow diversion valves 226 (flow diversion lines arenot shown here), and thence to the outgoing or raw product flow path ofa second regenerator 228. The second regenerator heats the milk (orother product) by heat exchange with the returning pasteurized product,so that the product leaving the regenerator 228 is a few degrees belowthe predetermined UHT pasteurizing temperature. Here, the leavingtemperature is 264° F., which is sixteen degrees below the UHTpasteurizing temperature of 280° F. The product leaving the raw side ofthe regenerator 228 enters a heater 230 where the product is heated to280°. The heater 230 is preferably a medium-to-product heat exchangerwhere hot water heated to 284° F. enters the heater 230 and flows incountercurrent with the product, and exits at 279° F. A steam-heated hotwater circulating unit 232 provides the hot water at the prescribedtemperature and at a predetermined flow rate. The rate of flow of thehot water through the heater 230 should be kept below three times therate of flow of product, and favorably about twice the rate of flow ofproduct.

The product that exits the heater 230 flows through a holding tube 234,where it is held at the UHT temperature of 280° for a prescribed time,e.g., two seconds minimum. After that, the pasteurized product entersthe return side of the second heat exchanger 228, where it is cooled byheat exchange with the incoming raw product. In this embodiment, thepasteurized product enters the regenerator 228 at 280° F. and leaves at190° F. At the same time, the raw product enters the regenerator 228 at175° F. and leaves at 264° F. This means that the temperaturedifferences at the low temperature and high temperature ends of theregenerator 228, between the pasteurized and unpasteurized product, arefifteen degrees and sixteen degrees, respectively. The temperaturedifferentials at all reference points between the high and lowtemperature ends will also be approximately 15-16° F., and will in allevents be below 20° F.

The pasteurized product that exits the regenerator 228 then flowsthrough the pasteurized side of the first regenerator 216, where iscooled by the incoming raw product, and exits at a temperature of about60° F. Then the product passes through a cooler 236, i.e., a heatexchanger where the pasteurized product is cooled by ice water or a foodgrade synthetic coolant, and exits at about 40° F. or below. From here,the pasteurized product passes a diversion valve to flow to a fillertank or sterile filler equipment (not shown). A diversion linesterilizer 238 heats water for the diversion line that leads to thebalance tank, and can also be used for cooling the sterilizing waterfrom a high temperature (up to 250°) to below boiling.

One or more steam heaters 240 may be provided for heating sterilizingwater for sterilizing the raw side passages, and a regenerator bypassline 242 is provided around the first regenerator 216 for use duringstart up.

The regenerative heat exchanger 228 could be a triple-tube device of thetype shown in U.S. Pat. No. 3,386,497, where there are inner, middle,and outer tubes arranged coaxially, to define an inner flow passage, anouter annular flow passage, and a middle annular flow passage. Theheater 230 could also be a triple-tube heat exchanger with the milk orother product flowing in the annular space between the inner and middletubes, and the hot water or other medium flowing counter-currently inthe outer and inner spaces.

The pre-existing HTST pasteurizer 210 may employ tubular heatexchangers, or may be of the type employing a plate or press heatexchanger for the regenerator 216.

In the UHT pasteurizer arrangement as described here, the regenerationefficiency of the UHT stage is a minimum of 81%, and where a tubularheat exchanger is employed also as the first stage regenerator 216, thetotal UHT or aseptic pasteurizer arrangement can have a regenerationefficiency exceeding 85% for liquids such as whole milk. For moreviscous products such as ice cream mix, UHT or aseptic pasteurizationcan have a regeneration efficiency of 75%. The equipment employed hererequires only standard dairy tubing and fittings, which only need to berated for up to 300 psi. The denaturizing of the milk or other foodproduct prior to going to the UHT stage, and homogenizing the milk at anelevated intermediate temperature prior the UHT stage prevent milksolids from depositing on the heat exchange surfaces in the heater or inthe regenerator. This improves the regeneration, due to absence ofsolids that would limit or inhibit heat exchange. Also, because thesolids do not deposit, the equipment can run for extended times beforemaintenance is required.

While the invention has been described with reference to a few selectedembodiments, it should be recognized that the invention is not limitedto those precise embodiments. Rather, many modifications and variationswill be apparent to persons skilled in the art without departing fromthe scope and spirit of this invention, as defined in the appendedclaims.

1. Process for ultrahigh temperature pasteurization of a liquid foodproduct using a pasteurization arrangement having a firstproduct-to-product regenerative heat exchanger, a first heater stage, asecond product-to-product heat exchanger, and a UHT heater stage;comprising the steps of supplying said liquid food product through araw-product side of said first regenerative heat exchanger to pre-heatsame; heating said preheated liquid food product exiting the first heatexchanger to a predetermined intermediate temperature suitable fordenaturizing proteins in the liquid food product; flowing the liquidfood product at said intermediate temperature through a timing tube tohold the product at said intermediate temperature for a predeterminedtime suitable for denaturing said proteins therein; flowing said liquidfood product from said timing tube through a raw-product side of saidsecond regenerative heat exchanger to preheat the same from saidintermediate temperature to a temperature near a UHT pasteurizingtemperature; flowing the liquid food product exiting said second heatexchanger into a medium-to-product heater in said UHT heater stage toheat the liquid food product to a predetermined UHT temperature, the UHTheater stage including means supplying a heating medium in counterflowto said product through said medium to product heater; holding theliquid food product exiting said medium-to-product heater at said UHTtemperature for a predetermined length of time; flowing the liquid foodproduct through a pasturized side of said second regenerative heatexchanger in counterflow to the liquid food product flowing in the rawside thereof to transfer heat to the product flowing in the raw sidethereof; flowing the liquid food product exiting the pasteurized side ofthe second regenerative heat exchanger through a pasteurized side of thefirst regenerative heat exchanger in counterflow to the liquid foodproduct flowing in the raw side thereof to transfer heat to the productflowing in the raw side thereof; and further processing the liquid foodproduct leaving the pasteurized side of the first regenerative heatexchanger to prepare same for packaging; wherein a temperaturedifferential in the medium-to-product heater between the liquid foodproduct and said heating medium at any point of reference in the heateris less than 20 degrees F.
 2. The process for ultrahigh temperaturepasteurization according to claim 1 wherein the liquid food product hasa product volume rate of flow through said medium to product heater andsaid medium has a medium volume rate of flow through said heater, andthe ratio of the medium rate of flow to the product rate of flow isbelow 3:1.
 3. The process for ultrahigh temperature pasteurizationaccording to claim 1 wherein said ratio of medium to product flow ratesis about 2:1.
 4. The process for ultrahigh temperature pasteurizationaccording to claim 1 wherein the temperature differential in themedium-to-product heater between the product leaving and the mediumentering is about 5 degrees F.
 5. The process for ultrahigh temperaturepasteurization according to claim 1 wherein the temperature differentialin the medium-to-product heater between the product entering and themedium leaving is about 15 degrees F.
 6. The process for ultrahightemperature pasteurization according to claim 1 wherein the temperaturedifferential in the second regenerative heat exchanger between theproduct leaving the raw side and the product entering the pasteurizedside is about less than 20 degrees F.
 7. The process for ultrahightemperature pasteurization according to claim 1 wherein saidpredetermined intermediate temperature is substantially 175 degrees F.8. The process for ultrahigh temperature pasteurization according toclaim 7 wherein said predetermined time that said timing tube holds theproduct at said intermediate temperature is substantially sixty seconds.9. The process for ultrahigh temperature pasteurization according toclaim 1 further comprising passing said product through a homogenizerprior to flowing the same through the raw side of the secondregenerative heat exchanger.
 10. The process for ultrahigh temperaturepasteurization according to claim 6 wherein said first and secondregenerative heat exchangers are tube-in-tube counterflow heatexchangers.
 11. Process for ultrahigh temperature pasteurization of aliquid food product using a pasteurization arrangement having a firstproduct-to-product regenerative heat exchanger, a first heater stage, asecond product-to-product heat exchanger, and a UHT heater stage;comprising the steps of supplying said liquid food product through araw-product side of said first regenerative heat exchanger to pre-heatsame said preheated liquid food product exiting the first heat exchangerto a predetermined intermediate temperature suitable for denaturizingproteins in the liquid food product; flowing the liquid food product atsaid intermediate temperature through a device to hold the product atsaid intermediate temperature for a predetermined time suitable fordenaturing said proteins therein; flowing said liquid food product fromsaid device through a raw-product side of said second regenerative heatexchanger to preheat the same from said intermediate temperature to atemperature near a UHT pasteurizing temperature; flowing the liquid foodproduct exiting said second heat exchanger into a medium-to-productheater in said UHT heater stage to heat the liquid food product to apredetermined UHT temperature, the UHT heater stage including meanssupplying a heating medium in counterflow to said product through saidmedium to product heater; holding the liquid food product exiting saidmedium-to-product heater at said UHT temperature for a predeterminedlength of time; flowing the liquid food product through a pasturizedside of said second regenerative heat exchanger in counterflow to theliquid food product flowing in the raw side thereof to transfer heat tothe product flowing in the raw side thereof; flowing the liquid foodproduct exiting the pasteurized side of the second regenerative heatexchanger through a pasteurized side of the first regenerative heatexchanger in counterflow to the liquid food product flowing in the rawside thereof to transfer heat to the product flowing in the raw sidethereof; and further processing the liquid food product leaving thepasteurized side of the first regenerative heat exchanger to preparesame for packaging; wherein a temperature differential in themedium-to-product heater between the liquid food product and saidheating medium at any point of reference in the heater is less than 20degrees F.
 12. The process for ultrahigh temperature pasteurizationaccording to claim 11 wherein the liquid food product has a productvolume rate of flow through said medium to product heater and saidmedium has a medium volume rate of flow through said heater, and theratio of the medium rate of flow to the product rate of flow is below3:1.
 13. Process for ultrahigh temperature pasteurization of a liquidfood product using a pasteurization arrangement having aproduct-to-product regenerative heat exchanger and a UHT heater stage;comprising the steps of supplying said liquid food product through araw-product side of said regenerative heat exchanger to pre-heat same,including denaturizing proteins in the liquid food product, andpreheating the liquid food product from said to a temperature near a UHTpasteurizing temperature; flowing the liquid food product exiting saidregenerative heat exchanger into a medium-to-product heater in said UHTheater stage to heat the liquid food product to a predetermined UHTtemperature, the UHT heater stage including means supplying a heatingmedium in counterflow to said product through said medium to productheater; holding the liquid food product exiting said medium-to-productheater at said UHT temperature for a predetermined length of time;flowing the liquid food product through a pasturized side of saidregenerative heat exchanger in counterflow to the liquid food productflowing in the raw side thereof to transfer heat to the product flowingin the raw side thereof; and further processing the liquid food productleaving the pasteurized side of the regenerative heat exchanger toprepare same for packaging; wherein a temperature differential in themedium-to-product heater between the liquid food product and saidheating medium at any point of reference in the heater is less than 20degrees F.
 14. The process for ultrahigh temperature pasteurizationaccording to claim 13 wherein the liquid food product has a productvolume rate of flow through said medium to product heater and saidmedium has a medium volume rate of flow through said heater, and theratio of the medium rate of flow to the product rate of flow is below3:1.
 15. The process for ultrahigh temperature pasteurization accordingto claim 14 wherein said ratio of medium to product flow rates is about2:1.
 16. The process for ultrahigh temperature pasteurization accordingto claim 13 wherein the temperature differential in themedium-to-product heater between the product leaving and the mediumentering is about 5 degrees F.
 17. The process for ultrahigh temperaturepasteurization according to claim 13 wherein the temperaturedifferential in the medium-to-product heater between the productentering and the medium leaving is about 15 degrees F.
 18. The processfor ultrahigh temperature pasteurization according to claim 13 whereinthe temperature differential in the regenerative heat exchanger betweenthe product leaving the raw side and the product entering thepasteurized side is about less than 20 degrees F.
 19. (canceled)
 20. Theprocess for ultrahigh temperature pasteurization according to claim 13wherein the product flowing through said medium-to-product heater has aflow velocity of below about nine feet per second.
 21. The process forultrahigh temperature pasteurization according to claim 20 wherein saidflow velocity is no greater than about six feet per second.
 22. In a UHTpasteurizer arrangement comprising a balance tank containing a supply ofa liquid food product; a product-to-product regenerative heat exchangerarrangement having a raw product side through which the liquid foodproduct is flowed from the balance tank and a pasteurized side throughwhich pasteurized product is flowed in counterflow relation so that heatis transferred from the pasteurized product to the raw liquid foodproduct to preheat same; a UHT heater stage including amedium-to-product heat exchanger in which the liquid food productleaving the regenerative heat exchanger arrangement enters themedium-to-product heat exchanger where the product is heated to apredetermined UHT temperature, a holding tube in which the liquid foodproduct leaving the medium-to-product heat exchanger is held at said UHTtemperature for a predetermined time, and in which the liquid foodproduct leaving the holding tube enters the pasteurized side of theregenerative heat exchanger arrangement; and means further processingthe food product leaving the pasteurized side of the regenerative heatexchanger arrangement to prepare same for packaging; the improvementwherein there is a temperature differential in the medium-to-productheat exchanger between the liquid food product and said heating medium,which at any point of reference therein is less than 20 degrees F. 23.UHT pasteurizer arrangement according to claim 22 wherein the flow rateof product through the medium-to-product heat exchanger and the flowrate of medium therethrough are controlled such that the ratio of mediumrate of flow to product rate of flow is below 3:1.
 24. UHT pasteurizerarrangement according to claim 22 wherein the temperature differentialin the medium-to-product heat exchanger between product leaving andmedium entering is about 5 degrees F.
 25. UHT pasteurizer arrangementaccording to claim 22 wherein the temperature differential in themedium-to-product heat exchanger between product entering and mediumleaving is about 15 degrees F.
 26. UHT pasteurizer arrangement accordingto claim 22 wherein the regenerative heat exchanger arrangement includesfirst and second stages, and a heater stage situated between said firstand second stages, in which liquid food product leaving a raw side ofthe first stage is heated in a heater to temperature suitable fordenaturizing proteins in said liquid food product to prevent theproteins from depositing on walls of the second stage and of themedium-to-product heat exchanger and is held in a timing tube for apredetermined length of time before entering the second stage of theregenerative heat exchanger arrangement.
 27. UHT pasteurizer arrangementaccording to claim 26 wherein said predetermined temperature issubstantially 175 degrees F and said predetermined length of time issubstantially sixty seconds.
 28. UHT pasteurizer arrangement accordingto claim 22 in which said regenerative heat exchanger arrangementincludes at least one tubular heat exchanger in which an inner tube ismounted coaxially within an outer tube to define counterflow paths forsaid raw side and said pasteurized side.
 29. UHT pasteurizer arrangementaccording to claim 22 in which said medium to product heat exchangerincludes a tubular heat exchanger in which an inner tube is mountedcoaxially within an outer tube to define counterflow paths for saidmedium and for said liquid food product.
 30. UHT pasteurizer arrangementaccording to claim 22 in which the regenerative heat exchanger is formedas a three-tube heat exchanger with inner, middle, and outer tubesarranged coaxially.
 31. UHT pasteurizer arrangement according to claim22 in which the medium to product heat exchanger includes a three-tubetubular heat exchanger with inner, middle, and outer tubes that definean inner passage, an outer annular passage, and a middle annularpassage, with the product flowing in the middle annular passage and withthe medium flowing counter-currently in the inner and outer passages.